A.
Buspirone [BuSpar] is an anti-anxiety medication unrelated to most
typical "sedatives", such as Valium, Xanax, etc. Buspirone is effective for
so-called generalized or "free-floating" anxiety, but not for panic attacks.
As your question suggests, buspirone works via the brain chemical
(neurotransmitter) serotonin. While we don't know exactly what causes
anxiety on a chemical level, there is evidence that either too much or too
little serotonin is involved--the brain preferring a serotonin level that is
just right.

Some theories suggest that depression is due to too little
serotonin, whereas anxiety is due to too much. Buspirone is a kind of
"two-fer" when it comes to serotonin--that is, buspirone is a partial
agonist. This is a drug that acts as a kind of weak stand-in for the native
brain chemical. So, if the person has too much serotonin, buspirone competes
with this chemical and effectively dilute its effect on nerve cell target
sites called receptors. If the person has too little serotonin, buspirone
can step in and give these serotonin receptors a little kick, so to speak.
(In high doses, buspirone appears to have antidepressant properties).

So,
you can think of buspirone as ambidextrous with respect to serotonin. For
a much more detailed explanation of these mechanisms, see the paper by MS
Eison in the Journal of Clinical Psychopharmacology vol 10, ppl 26-30, 1990.